Acousto-magnetic (AM) technology has been widely used in anti-theft system. AM systems are typically used in detection panels, deactivators, AM tags etc. The detection panels emit pulses at a certain frequency, excite the active AM tags and cause the AM tags to emit strong resonating signals, which can be detected by detection panels, and, therefore, set off the alarm. Currently, there are two types of the AM tags: anti-theft permanent AM clamps and anti-theft AM labels. Both types of AM tags use amorphous as the resonator(s), and crystalline magnetic materials as the bias.
Based on the detach methods to take pin out, AM clamps can be classified into two categories: mechanical detach and magnetic detach. Current AM clamps, such as Sensormatic Electronics Corporation (SEC) manufactured under the trademarks SuperTag® and UltraGator™, must use the non-magnetic method, namely mechanical (contact) detachers, because the strong magnetic field from the magnetic detachers will cause magnetized direction changes in the bonded ferrite magnet bias, which damage the AM alarming unit constructed from resonators and bonded ferrite magnet bias. The magnetic field from a rare earth permanent magnet, in a magnetic detacher to attract spring piece in a magnetic locker, can reach to about 4000-6000 Gauss, while current AM clamps such as manufactured by SEC under the trademarks “SuperTag” and “UltraGator” use bonded permanent magnet bias with coercivity range only in about 2000-3000 Oe. Such bias is easy to be re-magnetized to new directions by the strong and curvature magnetic field from magnetic detachers which are deviated from the original magnetic direction along the length dimension of the resonator(s). The result of the re-magnetization of the AM alarming unit is that the unit degraded from active to deactivated, which is a permanent damage to the hard tag. Thus, mechanical detachers have to be used for prior AM clamps.
Compared to non-contact magnetic detach, the mechanical detach process is more complicated, manufacture cost is higher, and the mechanical head is also easy to be damaged which results in higher maintenance costs. The mechanical detach has to be a contact method which is inconvenient for cashiers to operate. These disadvantages limit the further cost reductions, as well as limit market expansion of AM hard tags. Current AM alarm units use permanent magnetic materials, such as ferrite magnets, with a coercivity above 1000 A/m (125 Oe), with the resonators and bias pieces being stacked in different compartments. So far, only SEC can produce the qualified integrated AM unit while no other hard tag companies can make qualified resonators and bias. Therefore, these hard tag companies would have to purchase such alarming units first, then place the units into the hard tag shells to make hard tags. But, there is no such separated permanent AM alarming unit available in the market now. Consequently, some hard tag companies have to use an alternative way: to use disposable AM labels to act as permanent AM units to make hard tags. Current disposable labels use semi-hard magnetic material, that has direct current coercivity between 1000 A/m (12.5 Oe) and 10000 A/m (125 Oe), such as SEC manufactured DR labels. The original purpose for such disposable labels was to use it with active/deactivate functions. Deactivated labels will not set off the alarm at store exits. However, the hard tags made with disposable labels will have a fatal defect in that these kind of hard tags will become functionless when disposable labels are easily deactivated.
Known prior art AM labels, such as are disclosed in U.S. Pat. No. 6,359,563, are depicted in FIGS. 1 and 2 and include in an elongated plastic housing 101 and the housing cover 103, which is made of, from top to bottom, a cover film 105, double tape 106, semi-hard magnetic bias piece 104 and another piece of cover film all formed in a layered structure. Inside the housing, there is at least one or more resonator(s) 102 with the size matching the size of the house. The resonator(s) 102 has a bowed shape, while the bias piece 104 has a parallelogram shape or the shape of a parallelogram with corners cut. Prior AM labels use semi-hard materials to make the bias piece. If one has to use a DR label as the alarming unit to make permanent AM clamps, this AM clamp will become functionless because a magnet could change the DR performance or eliminate the effective magnetization of the AM label, causing a failure to alarm in the interrogation zone when the merchandises attached with such AM clamp are passing through the stores' gates.